UN envoy to Syria Staffan de Mistura has condemned the reported killing of some 71 civilians by barrel bombs dropped by government helicopters in two attacks in the northern Aleppo Province.

"The news of aerial bombing by Syrian helicopters on a civilian market area of the Aleppo neighborhood of Al-Shaar deserves the most strong international condemnation," de Mistura said in a statement after the May 30 attacks.

A monitoring group in Syria says the barrel bombs also wounded dozens.

The British-based Syrian Observatory for Human Rights (SOHR) said on May 30 that at least 59 people died at a market in Al-Bab, a town controlled by the Islamic State (IS) group.

Twelve were killed in Al-Shaar, a rebel-held neighborhood in Aleppo.

Barrel bombs are crude weapons made of oil drums, gas cylinders, or water tanks packed with explosives and scrap metal that are usually dropped from helicopters.

Syrian President Bashar al-Assad has rejected reports that his forces drop barrel bombs.

But SOHR says it has documented 17,000 barrel bombings by the Syrian regime since October 2014.